Conjugated Macrocycles Related to the Porphyrins. 21. Synthesis, Spectroscopy, Electrochemistry, and Structural Characterization of Carbaporphyrins

Abstract

The "3 + 1" variant of the MacDonald condensation has been shown to be an excellent methodology for synthesizing carbaporphyrins. In particular, 1,3-indenedicarbaldehyde condenses with tripyrranes in the presence of TFA to give, following oxidation with DDQ, a series of benzocarbaporphyrins in excellent yields. Triformylcyclopentadienes also afford carbaporphyrin products, albeit in low yields ranging from 5 to 8%. These hybrid bridged annulene structures have porphyrin-like electronic absorption spectra with strong Soret bands near 420 nm and a series of Q-bands through the visible region. The proton NMR spectrum confirms the presence of a strong diamagnetic ring current, and the meso-protons show up at 10 ppm, while the internal CH is shielded to approximately -7 ppm. Carbaporphyrins undergo reversible protonation with TFA. Initial addition of acid affords a monocation, although mixtures of protonated species are observed in the presence of moderate concentrations of TFA. However, in the presence of 50% TFA a C-protonated dication is generated. The dications relocate the pi-delocalization pathway through the benzo moiety of benzocarbaporphyrins, and these therefore represent bridged benzo[18]annulenes, although they nevertheless retain powerful macrocyclic ring currents. Carbaporphyrins with fused acenaphthylene and phenanthrene rings have been prepared, and the former demonstrated significantly larger bathochromic shifts in UV-vis spectroscopy that parallel previous observations for acenaphthoporphyrins. A diphenyl-substituted benzocarbaporphyrin 19b was also characterized by X-ray crystallography, and these data show that the macrocycle is reasonably planar although the indene subunit is tilted out of the mean macrocyclic plane by 15.5 degrees. The structural data indicates that the preferred tautomer in the solid state has the two NH's flanking the pyrrolene unit in agreement with previous spectroscopic and theoretical studies. Cyclic voltammetry for carbaporphyrin 19a was more complex than for true porphyrins, showing five anodic waves and two quasi-reversible reductive couples.

Adaptation of the Rothemund Reaction for Carbaporphyrin Synthesis: Preparation of meso-Tetraphenylazuliporphyrin and Related Benzocarbaporphyrins

“…These data are consistent with 11 a favoring the specific tautomer shown in Scheme where the two pyrrole hydrogens flank a central pyrrolene nitrogen rather than the alternate species 11 a′ . This tautomeric preference has also been noted for carbaporphyrins 1 8c,8g. The plane of symmetry for 11 a was confirmed by 13 C NMR spectroscopy.…”

“…The UV/Vis spectrum for 11 a (Figure 4a) shows a Soret band at 446 nm and a series of four Q bands that extend beyond 700 nm. As had previously been reported for meso ‐unsubstituted carbaporphyrins 1 ,8c,8g addition of TFA to solutions of 11 a leads to the formation of monocations and at higher acid concentrations C‐protonated dications are generated. The major formyl carbaporphyrin product 11 b gave similar spectroscopic properties, although the proton NMR spectrum was far more complex due to the lack of symmetry in this structure (Figure 7).…”

“…The new carbaporphyrins show strong ring currents, although once again these are not quite as large as those observed for the meso ‐unsubstituted series 8c,8g. In the 400 MHz proton NMR spectrum of 11 a (Figures 5 and 6), the internal CH is observed at δ =−5.3 ppm, while the external pyrrolic protons resonate between 8.5 and 8.8 ppm.…”

“…The broadening of the NH varied somewhat with concentration, but also sharpened up as the temperature of the NMR solution was lowered to −50 °C (Figure 5). Unlike meso ‐unsubstituted benzocarbaporphyrins,8c,8g the chemical shift for the internal CH resonance was not affected by concentration or temperature, possibly because the meso ‐substituents inhibit aggregation in solution. In addition, no other changes are observed in the NMR spectrum for 11 a at lower temperatures.…”

Metalation of Carbaporphyrinoid Systems

“…[22,33,34] 3-Hydroxy-2,6-pyridinedicarbaldehyde ( 14; X = N) reacted with tripyrranes 10 under the unusual '3 + 1' conditions to give the aromatic pyridone-containing macrocycle in excellent yields. [34] Other aromatic type A systems include benzocarbaporphyrins 5, [24,35] carbaporphyrins 16 [23,24] and carbachlorins 7, 26 all of which can be formed by reacting the necessary dialdehyde precursors with tripyrranes 10 (Scheme 1). The use of indene dialdehyde 17 in this chemistry to generate benzocarbaporphyrins 5 has been particularly successful.…”